1. Field Of The Invention
The present invention relates generally to downhole tools for use in oil and gas wells, and more particularly, but not by way of limitation, to downhole tools operated in response to a change in well annulus pressure.
2. Brief Description Of The Prior Art
The prior art includes many downhole tools which are operated in response to annulus pressure. One system utilizes a compressed nitrogen gas spring within the tool to accommodate displacement of a differential pressure piston which moves in response to changes in well annulus pressure. The differential pressure across the power piston is developed by placing a fluid flow restriction in a flow channel communicating one side of the power piston with the well annulus, while there is free communication between the other side of the power piston and the well annulus. Typical examples of such a system as applied to a combination closure and circulation valve are shown in U.S. Pat. No. 4,691,779 to McMahan et al., and U.S. Pat. No. 4,311,197 to Hushbeck.
One disadvantage of tools of the type utilizing a compressed nitrogen gas spring within the tool is that the nitrogen gas must be placed in the tool under pressure before the tool is lowered into a well.
One attempt to eliminate the need for pressurizing the tool has been to replace the compressed nitrogen gas spring with a compressed liquid spring, typically silicone oil. Typical examples of annulus pressure responsive tools utilizing a silicone oil liquid spring are seen in U.S. Pat. Nos. 4,448,254 and 4,444,268 to Barrington.
One disadvantage of tools utilizing a compressible silicone oil spring chamber to accommodate displacement of the differential pressure power piston is that a relatively large volume of silicone oil must be provided to provide sufficient volume change at typical operating pressures to accommodate the displacement of the power piston. These large oil volumes make the tool undesirably large, heavy and bulky to handle.
Thus, there is a need for an annulus pressure responsive tool actuation system which avoids both the necessity of pressurizing the tool internally with high pressure gas while also avoiding the extremely large tool sizes which have previously been necessary to accommodate compressible silicone oil liquid springs.